1. Field of the Invention
The present invention relates particularly to a heat spreader that can be suitably used for transferring heat from a semiconductor element required of high heat transfer characteristics, for example, a high-power semiconductor laser for laser beam machining, and to a semiconductor device using the heat spreader.
2. Description of Related Art
In order to transfer heat generated when semiconductor elements, represented by, semiconductor light emitting elements such as semiconductor lasers and light emitting diodes, operate outward from the elements, and more specifically into environments or the like through second-components such as heat sinks or stems, as heat spreaders such as sub-mounts, heat sinks or heat transferring substrates that are interposed between the semiconductor elements and the second-components, ones composed of Si, ceramics, etc. have been conventionally widely used. The reason for this is that the heat spreaders composed of Si, ceramics, etc., are easy to process, for example, to cut, are easy to perform pattern-forming for wiring to the semiconductor elements, and can keep manufacturing costs low (see, for example, Japanese Unexamined Patent Publication Nos. JP 06-350202 A (1994) and JP 2003-46181 A).
FIG. 15 is a sectional view showing an example of a conventional semiconductor device 4, configured by connecting a semiconductor laser 2 serving as a semiconductor element and a heat sink 3 serving as a second-component to a sub-mount 1 serving as the above-mentioned conventional heat spreader, for transferring heat generated when the semiconductor laser 2 operates to the heat sink 3 through the sub-mount 1 to transfer the heat into an environment. Referring to FIG. 15, the sub-mount 1 in the semiconductor device 4 in this example includes a plate shaped substrate 7 made of Si or ceramics as described above, and in which an upper surface and a lower surface of the plate respectively serve as an element connection surface 5 for connection of the semiconductor laser 2 and a second-component connection surface 6 for connection of the heat sink 3.
Furthermore, a region, to which the semiconductor laser 2 is connected, of the element connection surface 5 of the substrate 7 is coated with an element connecting layer 8 composed of a solder, and the second-component connection surface 6 is coated with a second-component connecting layer 9 composed of a solder having a melting point lower than that of the solder forming the element connecting layer 8. Further, the element connection surface 5 of the substrate 7 may, in some cases, be coated with a metal layer such as an Au layer, which is not illustrated, by sputtering, vacuum deposition, wet plating, or the like prior to coating the element connecting layer 8 in order to provide the element connection surface 5 with good wettability with the solder forming the element containing layer 8.
The semiconductor laser 2 is formed by laminating a plurality of semiconductor layers, electrode layers, or the like in a vertical direction, and its side surface crossing the direction of the lamination serves as an emission surface 12 for emitting laser light 11 generated in an active layer 10 provided halfway in the direction of the lamination. The heat sink 3 releases heat generated when the semiconductor laser 2 operates and transferred through the element connecting layer 8, the sub-mount 1 and the second-component connecting layer 9 into an environment or further transfers the heat to a third-component, and is generally formed of Cu or the like.
The semiconductor device 4 in the example illustrated in FIG. 15 is manufactured in the following procedure, for example. That is, the sub-mount 1 and the semiconductor laser 2 are connected to each other by heating the sub-mount 1 and the semiconductor laser 2 in a laminated state with the element connecting layer 8 sandwiched therebetween to melt the solder forming the element connecting layer 8, thereby to join the element connecting layer 8 to a lower surface of the semiconductor laser 2 with soldering. Then, when the sub-mount 1 and the heat sink 3 are connected to each other by heating the sub-mount 1 and the heat sink 3 to a temperature that is not more than the melting temperature of the solder forming the element connecting layer 8 and is not less than the melting temperature of a solder forming the second-component connecting layer 9 in a laminated state with the second-component connecting layer 9 sandwiched therebetween to selectively melt the second-component connecting layer 9, thereby to join the second-component connecting layer 9 to an upper surface of the heat sink 3 with soldering, the semiconductor device 4 is manufactured.
The sub-mount 1 and the semiconductor laser 2 are generally connected to each other with the emission surface 12 and a side surface 13 on the same side of the emission surface 12 out of side surfaces, crossing the element connection surface 5 and the second-component connection surface 6, of the substrate 7 in the sub-mount 1 aligned so as to be flush with each other as illustrated, in order to prevent a luminous flux of the laser light 11 emitted from the emission surface 12 from being blocked by the sub-mount 1. However, when the sub-mount 1 and the semiconductor laser 2 are connected to each other, the solder forming the element connecting layer 8 may, in some cases, be melted by heating and solidified with the solder jutting out to the emission surface 12. In the case, the solder that has jutted out may block the luminous flux of the laser light 11 emitted from the emission surface 12 and may short-circuit the semiconductor laser 2.
Therefore, JP 06-350202 A (1994) previously described, JP 05-243690 A (1993), or the like discloses that the metal layer such as the Au layer for providing good wettability with the solder is formed so as to connect with not only the element connection surface 5 but also the side surface 13, to also provide the side surface 13 with good wettability with the solder, and the solder that has jutted out by being melted when the sub-mount 1 and the semiconductor laser 2 are connected to each other is guided so as to flow toward the side surface 13 by the function of the metal layer, thereby to prevent the solder from jutting out into the light emission surface 12. In order to obtain the same effect, JP 2003-46181 A or JP 08-330672 A (1996) discloses that a solder layer is previously formed on the metal layer formed on the side surface 13 so as to continue with the element connecting layer 8.
In recent years, a large current must be cause to flow in a semiconductor element such as a semiconductor laser particularly as the power of the semiconductor element increases. Therefore, an attempt to form a substrate of a heat spreader such as a sub-mount of a material containing a metal and having conductive properties and make the substrate itself also serve as an electrode of the element has been made. JP 2003-152145 A, for example, discloses that a substrate of a heat spreader such as a sub-mount is formed of a Cu—W composite material having a composite structure formed by infiltrating Cu into a pore of a porous body composed of W.
Even when a heat spreader in which a substrate is formed of a material including a metal such as the Cu—W composite material described above (which may be hereinafter referred to as a “metal-containing material”) is used to compose a semiconductor device, the basic structure thereof is generally as illustrated in FIG. 15. It is considered that a structure for preventing a solder melted when the sub-mount 1 serving as a heat spreader and the semiconductor laser 2 serving as a semiconductor element are connected to each other from jutting out into the emission surface 12 of the semiconductor laser 2 is basically the same as the conventional structure.
However, the substrate 7 composed of the metal-containing material is itself superior in wettability with the solder to the conventional one composed of Si, ceramics, or the like. When the sub-mount 1 is connected to the heat sink 3, therefore, the solder forming the second-component connecting layer 9 may creep up along the side surface 13 of the substrate 7 to the vicinity of the emission surface 12 of the semiconductor laser 2 with the solder melted by heating, thereby to easily short-circuit the semiconductor laser 2 and block the luminous flux of the laser light 11 emitted from the emission surface 12.
Particularly when a solder superior in flowability at low temperature or a paste-shaped or foil-shaped solder, which is superior in the effect of improving yield in manufacturing the semiconductor device 4 because solder wetting defects such as a void preventing heat transfer may hardly occur so that a good solder joint can be obtained, is used as the solder forming the second-component connecting layer 9, the above-mentioned defects easily occur. The reason why the above defects can easily occur when the paste-shaped or foil-shaped solder is used is that the amount thereof is not easily controlled, so that an excessive amount of melted solder is easily produced.